Wireshark-dev: Re: [Wireshark-dev] TCP dissect issue when app-level message spans multiple TCP
From: "Fernandez, Rafael" <[email protected]>
Date: Thu, 5 May 2011 12:39:30 -0500
OK. This function returns exactly the same as yours. The rest of the code in there for debugging purposes. I appreciate you trying to help me but you are focusing in wireshark coding standards and lines that do not have anything to do with the issue I am experiencing. The issue is the following:

In epan/dissectors/packet-tcp.c-tcp_dissect_pdus():
line 1993: get_pdu_len returns 322. Sets plen to 322.
line 2053-2061: length_remaining is 144. Thus (length_remaining < plen) is true. Sets pinfo->desegment_offset and pinfo->desegment_len. tcp_dissect_pdus returns.

In my_dissector.c:
1. Once tcp_dissect_pdus returns, dissect_message returns.
2. dissect_message gets called soon after and calls tcp_dissect_pdus again.

In packet-tcp.c-tcp_dissect_pdus:
Line 1993: get_pdu_len gets called again. It read 4 bytes starting from the 178th (322-144) byte of the application message. Thus, it returns a garbage size.

Don't you think there is an issue with this?


-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Chris Maynard
Sent: Thursday, May 05, 2011 11:27 AM
To: [email protected]
Subject: Re: [Wireshark-dev] TCP dissect issue when app-level message spans multiple TCP packets

Fernandez, Rafael <[email protected]> writes:

> This is my current get_message_tcpmessage_len:
>
> guint get_message_tcpmessage_len(packet_info *pinfo, tvbuff_t *tvb, int offset)
> {
>         guint remaining  = tvb_length_remaining(tvb, offset);
>         guint last_size = tvb_get_letohl(tvb, offset)+MESSAGE_HEADER_SIZE;
>         if(last_size > remaining)
>         {
>                 printf("not enough data: %d remaining: %d\n", last_size,
remaining);
>         }
>         return last_size;
> }
>

This is still wrong.  You shouldn't be trying to figure out if you've got enough
data; tcp_dissect_pdus() will handle all that for you.  This function only needs
to return the length of the entire PDU so tcp_dissect_pdus() knows how much data
it needs to reassemble before calling your dissector.  Try changing the function
to something like what I posted earlier.

You might also re-read section 2.7.1 of doc/README.developer, as it could help
you in the case of UDP.  And take a look at other examples in the Wireshark
codebase, such as packet-dns.c which doc/README.developer references in its
example.

And stop using printf().  If you need to, try using g_warning() instead.

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